1. Field of the Invention
The present invention relates to an ink jet printing apparatus and an ink jet printing method for printing an image with reciprocating scanning of a printing head capable of ejecting ink.
2. Description of the Related Art
In recent years, an ink jet printing apparatus which performs printing by ejecting ink onto a printing medium from nozzles of a printing head has been widely used as an apparatus for outputting an image created by a computer or an image taken by an image pickup device such as a digital camera. This ink jet printing apparatus can form a high-quality image comparable to a silver halide photograph by use of a small and inexpensive configuration. Currently, there is also available a printing apparatus capable of printing an image on an entire surface of a printing medium without leaving any margin at the ends of the printing medium as in the case of the silver halide photograph.
To improve a printing speed, such an ink jet printing apparatus employs a printing head having a plurality of ink ejection ports and liquid passages integrated therein, as a printing head (hereinafter also referred to as a “multi-head”) having a plurality of printing elements integrated and arranged therein. Furthermore, an ink jet printing apparatus capable of printing a color image generally uses a plurality of such multi-heads.
A so-called serial scan type ink jet printing apparatus prints images sequentially on a printing medium by repeating printing scanning of such a printing head in a main scanning direction and transfer movement of the printing medium in a sub-scanning direction intersecting the main scanning direction. The printing head is usually mounted on a carriage capable of reciprocating movement along the main scanning direction, and ejects ink while moving in the main scanning direction together with the carriage during the printing scanning. There are two types of methods for printing images: one is a one-way printing method for performing printing scanning in movement of the printing head only in one direction, and the other is a two-way printing method for performing printing scanning in movement of the printing head in one direction and in the other direction.
An ink droplet to be ejected from the ink ejection port of the printing head include a main droplet and a small droplet separated out of the main droplet. The main droplet and the small droplet form large dot and small dot, respectively, when landing on the printing medium. The small dot is also called a “satellite”. The small droplet forming the satellite is ejected simultaneously with the main droplet. Specifically, during ink ejection, a main droplet has a tail portion caused at its rear side by a tension between the main droplet and a liquid level of an ink meniscus in the ink ejection port. Then, the tail portion is separated by a surface tension so as to form a spherical shape. Thus, the small droplet is formed. As described above, the surface tension acting when the small droplet is separated from the ink meniscus in the ink ejection port pulls the small droplet backwardly in an ejection direction. Thus, an ejection speed of the small droplet is slower than that of the main droplet.
Moreover, in the case where a printing surface of the printing medium is parallel to an opening surface (ejection port forming surface) in which the ink ejection ports are formed, a relationship between landing positions of the main droplet and the small droplet, which are different from each other in the ejection speed, is constant as long as the opening surface is even. Thus, except for the case of a significant temperature increase or the like, quality of printed images is unlikely to be changed even in the two-way printing method.
However, when the ink ejection ports are formed to be tilted with respect to the opening surface, the opening surface around the ink ejection port may have a partially-varying affinity for ink, so that the ejection direction of the small droplet is changed.
FIGS. 6A and 6B are explanatory views showing different formation examples of ink ejection port arrays (hereinafter also referred to as “nozzle arrays”) in printing heads H. In FIGS. 6A and 6B, No denotes odd-numbered (N1, N3, . . . ) ink ejection ports (hereinafter also referred to as “odd nozzles”) from one end of the nozzle array, and Ne denotes even-numbered (N2, N4, . . . ) ink ejection ports (hereinafter also referred to as “even nozzles”) from the one end of the nozzle array. In the printing head H shown in FIG. 6A, the odd nozzles No and the even nozzles Ne are formed at equal intervals on one nozzle array. Meanwhile, in the printing head H shown in FIG. 6B, the odd nozzles No are formed at equal intervals (pitches P) on an odd nozzle array Lo and the even nozzles Ne are formed at equal intervals (pitches P) on an even nozzle array Le. Moreover, those nozzles No and Ne are shifted from each other by a half pitch (P/2).
In the printing head H as described above, when the odd nozzles No are formed to be tilted toward one side in main scanning directions and the even nozzles Ne are formed to be tilted toward the other side in the main scanning directions, a relationship between landing positions of main droplets and small droplets may change depending on scanning directions as described below.
FIGS. 7A and 7B are explanatory views showing landing positions of main droplets and small droplets, which are ejected from the printing head H shown in FIG. 6B. A main droplet and a small droplet, which are ejected from the even nozzle Ne, form a main dot D′1 and a satellite D′2, respectively, on a printing medium P. Moreover, a main droplet and a small droplet, which are ejected from the odd nozzle No, form a main dot D1 and a satellite D2, respectively, on the printing medium P. In the case of this example, the even nozzle Ne is formed to be tilted to a forward direction (first direction) X1 in the main scanning directions, and the odd nozzle No is formed to be tilted to an opposite direction (second direction) X2 in the main scanning directions. Moreover, the tilt of the even nozzle Ne to the forward direction X1 is equal to the tilt of the odd nozzle No relative to the opposite direction X2.
FIG. 7A is the explanatory view showing the case of printing scanning in the forward direction X1, and FIG. 7B is the explanatory view showing the case of printing scanning in the opposite direction X2. In each of FIGS. 7A and 7B, VD1 is an ejection speed of the main droplet that forms the main dot D1, VD′1 is an ejection speed of the main droplet that forms the main dot D′1, VD2 is an ejection speed of the small droplet that forms the satellite D2, and VD′2 is an ejection speed of the small droplet that forms the satellite D′2. The ejection speeds VD2 and VD′2 of the small droplets are slower than the ejection speeds VD1 and VD′1 of the main droplets. Moreover, ejection directions of the small droplets are shifted from those of the main droplets under the influence of ink affinity of the opening surface (ejection port forming surface) H1 of the printing head H.
Since the main droplets D1 and D′1 and the small droplets D2 and D′2 are ejected during movement of the printing head H, a movement speed of a carriage moving together with the printing head H is added to the ejection speeds of the droplets. Thus, when a movement direction of the carriage is the same as the ejection direction of the ink droplet (the tilt direction of the ejection port), landing positions of the main droplets and the small droplets are shifted from each other so as to form the main dot D′1 and the satellite D′2 in FIG. 7A and the main dot D1 and the satellite D2 in FIG. 7B. Specifically, the landing positions of the small droplets are shifted in the movement direction of the carriage from those of the main droplets. Meanwhile, when the movement direction of the carriage is opposite to the ejection direction of the ink droplet, the main droplets and the small droplets land on approximately the same positions so as to form the main dot D1 and the satellite D2 in FIG. 7A and the main dot D′1 and the satellite D′2 in FIG. 7B.
As described above, the change in the relationship between the landing positions of the main droplets and the small droplets depending on the scanning directions may impair quality of printed images when the two-way printing method is employed.
Japanese Patent Laid-Open No. Hei 8 (1996)-58083 describes a configuration with a printing head having all the ink ejection ports tilted in the same direction, in which a printing scanning speed between printing scanning in a forward direction and that in an opposite direction is changed according to a tilt of ink ejection ports in order to suppress such a change in a relationship between landing positions of main droplets and small droplets. Moreover, Japanese Patent Laid-Open No. 2006-168374 describes a configuration in which printing scanning in the forward direction and that in the opposite direction as shown in FIGS. 7A and 7B are repeated in a multi-pass printing method for printing in a predetermined printing region on a printing medium by scanning more than once. In the case of Japanese Patent Laid-Open No. 2006-168374, a visually good image can be printed, regardless of a change in a relationship between landing positions of main droplets and small droplets, by changing a transfer amount of the printing medium between the printing scanning in the forward direction and that in the opposite direction.
However, the configuration described in Japanese Patent Laid-Open No. Hei 8 (1996)-58083 changes the printing scanning speed between the printing scanning in the forward direction and that in the opposite direction according to the tilt of the ink ejection port (tilt of the ink ejection direction). Thus, throughput degradation may occur. Moreover, the configuration described in Japanese Patent Laid-Open No. 2006-168374 changes the transfer amount of the printing medium between the printing scanning in the forward direction and that in the opposite direction. Thus, transfer control of the printing medium may become complicated.